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Palaeolv@D@U@A P@DMOGA Discussion Vol AWA PAlAEOlV@D@U@A P@DMOGA Discussion Vol. 38, No. 3/4, pp. 273-280, Warszawa, 1994 Candle wax shells, morphodynamics, and the Cambrian explosion ADOLF SEILACHER What I miss most with Antoni Hoffman is the ideal discussant for trying out new and often controversial ideas: very sharp and critical, yet warm- hearted enough not to become polemic. The last time we met, I told him about our candlewax shells and their implications for the emergence of new bauplans (Seilacher & Klug 1993). ~ur~risin~l~he reacted unlike the editors of a leading science journal who had just rejected my note without asking a referee. Instead he encouraged me by saying this was the deepest thought he had ever heard from me. Thank you, Toni! So here is the story again, hopefully in a more ordered form. Candle wax shells The experiment. - The crucial discovery was made by our Tubingen student Christian Klug, or rather his mother. During Christmas prepara- tions, she had placed a burning candle into a bowl of water. The bottom of the massive float that grew in the water from the molten wax (Fig. 1) resembled a real bivalve shell to the extent that even a specialist could be fooled. There was not only the familiar geometry of a spiral cone with large expansion rate; the shell also bore a hemispherical prodissoconch at the umbo and sets of growth lines mimicking daily and monthly increments. Repeating this unsophisticated experiment under controlled conditions, C. Klug and H. Luginsland produced hundreds of such wax shells. In spite of great variability, they show a general rule: populations grown in cold water have narrow and highly vaulted geometries and bear heavy growth lines (Gryphaea type; Fig. 2A-C). With rising temperatures, the expansion rate increases, outlines become more circular and growth lines fainter (Placunatype; Fig. 2H). Eventually, as the water reaches the melting point of the wax, shells switch into the morphospace of floating oildrops that we know from a good broth. Attempts to simulate , by the same procedure, fully coiled ammonite or gastropod shells did fail for practical reasons. The manipulations necess- ary (growth around a floating tooth stick; lateral shift of the wax source) invariably derailed the growth process. Minor translation (and low-degree helicoidality), however, did occur in variants in which a slight asymmetry 274 Discussion: SEILACHER happened to become enhanced during further growth of the float. Only patelliform cones could be produced by pouring the molten wax freely down on the water (Fig. 3A). Some of the occasional monsters tell stories of their own. If, for instance, the initial structure consists of many clustering drops instead of a single one, the accretionary spiral coils in proportion to the enlarged radius and starts with a sharp angle - as it does in oysters with broad attachment areas (Fig. 3B). Or, when a floating dirt particle has locally indented a growth line, it takes several more increments to restore the smooth curve -just like in mollusc shells 'healing' a marginal bite mark. Reductionistic Interpretation.- In line with our scientific tradition, we may try to identify the local controls responsible for corresponding features in the two systems: Wax float Bivalve shell Prodissoconch free pneu surface of initial drop replica of hydrostatic veliger larva wax flow freezes marginally in rhythmic growth phases of mant- small increments le are frozen by biomineralization Rounded contour 1 margin (Markov Effect) control by water table I control by margin of opposite valve flexible connection of float with flexible ligament, with tendency to candle causes tilting expand laterally for hinge function Helicoidality accidental overload on one anatomical anteroposterior ~rnside, self-enhancing differentiation differential immersion growth rates decline with age growth lines decreases with size In this view, candle wax shells only confirm what has already been known from computer simulations: regular shell geometries result from relatively simple sets of parameters and processes, whose variability defines a certain morphospace. But does a mere list of determining factors - however complete - do justice to the phenomenon as a whole? In an organism, individuality emerges because each shell is an integrated structure with a particular history. Why not in a physical system? Holistic Interpretation (Morphodynamics).- In the view of Gunji et al. (in press), biological systems are non-programmable in principle. Not that there is no program behind; but since informations propagate and are translated by enzymes (and therefore more slowly than in physical sys- tems), other signals or state changes can interfere during the process. As a result, the outcome can never be predicted in every detail. This means that computer simulation refers only to the basic, programmable part of a biological structure. The rest is either random or has the aspect of finality. ACTA PALAEONTOLOGICA POLONICA (38) (3/4) 275 Fig. 1. The lower surface of a self-organized candlewax float (B) could be easily mistaken for the cast of a real bivalve (A), complete with prodissoconch, daily and monthly growth lines and spiral cone geometry. Scale bar - 1 cm. Wax shells come closer to biological reality than computer simulations, because they imply unpredictability at the local level, historical (Marko- vim) self-reference and a finite size imposed by the dimensions of the experimental setting. In the above list, one feature of the wax floats deserves special attention: the growth lines. In an organism, we use to relate them to fluctuations in the physiological state, whether intrinsic or environmentally imposed. But wax shells form in minutes, under constant conditions and from a con- tinuous flow, comparable to lava flowing into the ocean. Thus the growth lines observed in either case reflect an intrinsic rhythm generated by the self-referent interaction between molten and frozen states. In such sy- nergetic oscillations (Haken 1983),it is also a familar phenomenon that higher-order periodicities emerge on top of the lower ones. The mantle/shell complex of a real mollusk forms a similar interactive system. During growth, biomineralization freezes the previous soft state whose shape was controlled by the mantle edge and a hydrostatic perios- tracum fold. At the same time, the frozen edge serves as a scaffold for the next growth ring. Since growth rings are also found in mollusc shells from the deep sea (i.e. in the absence of tidal, diurnal and seasonal forcing), rhythmic growth appears to be an intrinsic quality of biological skeletons 276 Discussion: SEILACHER - which does not exclude secondary linkage with external periodicities in particular environmental settings. This insight has consequences for sclerochronology; but more import- ant is the message that in morphogenesis one should not even think of skeletons and soft parts in isolation, because they form the necessary partners in a synergetic system. Nobody would challenge the statement that a mollusc or brachiopod shell is the product of the mantle. But the opposite is equally true in this self-referent system: the shell produces the mantle! The Cambrian turnover The sudden appearance of skeletonized animal phyla at the base of the Cambrian is perhaps the greatest riddle of the biohistorical record. In reality, it presents us with a whole bundle of problems: (1)the suddenness of the evolutionary event, (2) its synchroneity, (3) the ecological turnovers combined with it. Suddenness ('Cambrian explosion').- In the gradualistic view, ani- mal phyla had their roots far back in the Proterozoic, but became visible in the fossil record only with the acquisition of hard skeletons (e.g. Conway Morris 1990). If one assumes, however, that the impressions of large soft-bodied organisms of the Late Recambrian ('Ediacaran Fauna') repre- sent an extinct clade unrelated to modem animal phyla (Vendobionta hypothesis; Seilacher 1992a, b), this view looses considerable substance. On the other hand, behaviorally differentiated trace fossils tell us that small, worm-grade bilaterians with highly developed locomotory and neu- ral systems did burrow and forage underneath microbial mats long before the Cambrian revolution. Alternatively, the punctualistic view faces the old problem of 'hopeful monsters'; i.e. the emergence of new bauplans by a small number of serendipitous macromutations, for which known microevolutionary mech- anisms fail to provide actualistic models. If, however, bauplans (like many innovations at lower taxonomic levels; Seilacher 1991) arose largely by synergetic self-organization, their relatively sudden appearance becomes more acceptable. The synergetic view may also solve another problem. In recent years, molecular distances of DNA and its products in present-day organisms have increasingly provided us with a key to phylogenetic relationships, in addition to morphological similarities and the fossil record. According to molecular data, branching points between modem phyla appear to pre- date the Cambrian explosion (Bergstrom 1986 ); i.e. they favor a more gradualistic view. Synergetic innovation, however, may revolutionize mor- phological design without affecting the molecular clock. An Early Cambrian mollusc was probably close to its worm-like Precambrian ancestor in molecular terms, yet represented a fundamentally different world with regard to design and life style. Therefore, incongruence between molecular ACTA PALAEONTOLOGICA POLONICA (38)(3/4) 277 Fig. 2. Candlewax shell morphologies change consistently with water temperature. In cold water they tend to be narrow and highly convex with prominent growth lines (Gryphaeatype: A-C). At higher temperatures, shapes get wider and flatter, while growth lines become fainter (Placunatype; G-H). Above the melting point, wax floats switch into the morphospace of oil drops. Scale bar - 1 cm. and structural disparity should be expected, the first suggesting a more gradualistic, the other a punctuated mode of change. Synchroneity of skeletonization. - Firstly , the 'big bang' view of the Cambrian revolution needs to be relativated. In geohistoric perspective, 'sudden' may still mean a million years. Also, some skeletal phyla (e.g.
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